…yload`

Determining the error type when deserializing an `@httpPayload` is a
protocol-specific concern, and as such should not live in
`ServerHttpBoundProtocolGenerator`, which should remain
protocol-agnostic. This commits makes that determination part of the
`ServerProtocol` interface.

As a drive-by improvement, the companion object in
`ServerHttpBoundProtocolGenerator` has also been removed, since its
members have been unused for a long time.

…serialize-payload-error-type